1. Field of the Invention
The present invention relates to an apparatus and method for growing large, long, high-quality single crystals, and more specifically to an apparatus and method for maintaining a favorable thermal environment for the growth of longer single crystals in a liquid-encapsulated Czochralski (LEC) method.
2. Description of the Related Art
The well-known Czochralski or LEC method of growing single crystals involves contacting a seed crystal with a melt of the material to be drawn into a crystal, the melt being contained in a heated crucible. The growth of longer and larger diameter substantially defect-free crystals continues to be a goal pursued by those working in this field. Many difficulties and complications are encountered in attempts to improve upon the basic LEC process in order to grow the longer and larger diameter crystals, largely attributable to the melt thermal conditions, the thermal conditions in the crystal solidification zone, and the thermal environment in the upper portion of the vessel.
Various methods for controlling the axial thermal gradient in the crystal being grown have been reported in the art, and those methods generally involve the use of active or passive heat shields. Control of the axial thermal gradient through the use of heat shields is proposed to minimize stress and reduce dislocation density in the crystal being pulled. The use of such heat shields, however, has met with only limited success in practice, and the goal of achieving longer and larger defect-free crystals in a production setting has remained elusive.
It is therefore a principal object of the present invention to provide an apparatus and method which achieve the above-stated, and heretofore elusive, goal of being able to grow larger, longer, substantially defect-free single crystals.
It is another principal object of the present invention to provide an apparatus which broadens the process window for the growth of high quality, long single crystals by improving the control over the thermal environment in an LEC crystal puller.
It is a further principal object of the present invention to provide a method for controlling the thermal environment in an LEC crystal puller, which method has the effect of broadening the process window for the growth of high quality long single crystals.
It is an additional object of the present invention to provide an apparatus in which the cooling of an outer vessel is achieved by using two separate cooling circuits or fluid paths, a first of which cools the lower portion of the vessel where the crucible and heater are disposed, and a second of which cools the upper portion of the vessel to which the pulled crystal is exposed, and in which the cooling circuits can be used to control the direction of gas flow within the vessel.
It is an additional object of the present invention to provide an apparatus in which a stationary shield means is provided to surround the crucible or susceptor wall, above and at an inner and outer periphery thereof, which provides a more uniform thermal environment for the emerging crystal.
It is yet an additional object of the present invention to provide a method for controlling the direction of gas flow within the vessel and for controlling the thermal environment at the region of the emerging crystal, in order to provide overall favorable thermal conditions enabling the growth of larger and longer high quality, substantially defect-free single crystals.